Subject line
The text displayed at the top or beginning of an email, providing a concise summary and preview of the email's content. This line is visible in the recipient's inbox, influencing their decision to open or ignore the email.
Subject lines longer than 40 characters are often truncated on mobile and subject lines longer than 90 characters are often truncated on desktop. It's important to communicate to subscribers why they should open the email in less than 40 characters for best results.
Why the subject line matters
The subject line is the first, and often only, thing a subscriber reads before deciding whether to open. Together with the sender name and preheader text, it decides your open rate, which in turn feeds the engagement signals mailbox providers use for inbox placement. A weak subject line costs you opens; a misleading one costs you trust and spam complaints.
How to write a subject line that gets opened
- Lead with the payoff. Put the most useful words first, because mobile inboxes cut off after 30 to 40 characters.
- Be specific. "3 onboarding emails that cut churn 12%" beats "Our latest newsletter".
- Match the email. The subject line should describe what is inside. Clickbait raises opens once and lowers them after.
- Personalise carefully. A first name or company can lift opens, but only when it reads naturally. Segment-based relevance usually beats merge tags.
- Avoid spam patterns. All caps, stacked exclamation marks, "FREE!!!", and fake "Re:" prefixes trigger filters and irritate readers.
- Use the preheader. Treat it as the second half of the subject line rather than letting it default to "View in browser".
Subject line length by device
- Mobile: about 30 to 40 characters visible
- Desktop webmail: about 60 to 90 characters visible
- Sweet spot for B2B newsletters: 35 to 50 characters, with the key phrase in the first 30
Testing subject lines
Run an A/B test on a sample of your list, send the winner to the rest, and judge on clicks as well as opens. Apple Mail Privacy Protection inflates open rates, so a subject line that wins on opens but loses on clicks is not a winner. For examples and a deeper playbook, see email subject line best practices and our B2B subject line strategy guide.